Superfinishing large planetary gear systems

ABSTRACT

Disclosed herein is a new improved large planetary gear system used on the input stage of wind turbine power generators. This improved planetary gear system reduces or eliminates lubricant debris traditionally generated from the gear teeth, thereby eliminating an initiating source for bearing failure. To achieve these results, some and preferably all of the gear teeth within the planetary gear system are superfinished using chemically accelerated vibratory finishing to a surface roughness of approximately 0.25 micron or less. Several objects and advantages of the invention are to provide a gearbox with reduced metal debris, improved bearing life, reduced wear, reduced vibro-frictional noise, improved contact fatigue, improved fretting resistance, improved lubrication, to simplify the run-in process, and to enhance the durability and efficiency of the gearbox.

This application claims the filing date benefit of U.S. ProvisionalApplication No. 60/474,836, filed May 30, 2003, and also claims thefiling date benefit of U.S. Provisional Application No. 60/475,210,filed Jun. 2, 2003.

BACKGROUND OF THE INVENTION

This invention relates to a new improved input planetary stage for alarge gear box. The input planetary stage cited in this invention is fora wind turbine power generator having an output power capacity rating of500 kW and greater.

Wind turbine power generators are considered one of the most costeffective and environmentally friendly methods of generatingelectricity. Individual wind turbines are currently being designed andbuilt for electrical power generation in excess of 5 MW. A key componentof most wind turbines are their gearboxes, which are subjected tovarying high loading at low speeds, and have design lifetimes of 20years. Anything that will give these gearboxes more durability andefficiency is highly coveted by wind turbine manufacturers andoperators.

Modern large wind turbine generators (500 kW and greater) are massivedevices commonly using large planetary gear systems as the input stage.These heavy gearboxes, which are mounted atop high towers, often locatedin remote locations such as on a mountain or offshore, experience severefluctuations in wind conditions and temperature and are often exposed toa corrosive seawater environment and/or abrasive particulates. A gearboxfailure can require removing the gearbox using mammoth equipment, andrebuilding it back at the manufacturer's facility followed byreinstallation at the remote location. The concurrent loss of electricalgeneration is also a costly event unto itself.

Manufacturers recognize that removing the peak asperities from thecontact surfaces of gear teeth prior to full field operation increasesthe service life of the gearbox. There are two obvious advantages toremoving peak asperities. Firstly, this will reduce the amount ofmetal-to-metal contact which produces lubricant debris and which isknown to be destructive to gears and bearings. Secondly, it improves thematerial ratio (R_(mr)), which is a measure of the amount of gear toothsurface available for supporting the load. The industry assumed that anytechnique to remove the peak asperities was equivalent as long as noobvious metallurgical damage or no significant alteration to the leadand profile geometry occurred. Gear honing, for example, is often usedin the aerospace industry to reduce peak asperity heights. Honing couldhave been a consideration for wind turbine gearboxes; except that it iscost prohibitive on such large gears as most honing equipment is limitedto processing gears having a diameter of 12 inches or less. As such,today's wind turbine gears typically have ground teeth flanks and arerecommended to be operated through a run-in procedure to remove the peakasperities from the contact surfaces.

It has been taught for a number of years that optimum performancebenefits for bearings are achieved when the mating contacting surfacesare both isotropically superfinished to an arithmetic average roughness(R_(a)) of less than approximately 0.075 micron using chemicallyaccelerated vibratory finishing. Similarly, gears in auto racingtransmissions, which operate under high loads with high pitch linevelocities, have benefited from this isotropic superfinishing processwith teeth finishes of R_(a) from 0.3 micron to less than 0.025 micron.Such superfinished gears experience reduced contact fatigue, operatingtemperature, friction, noise and vibration.

Superfinishing enables the development of hydrodynamic lubrication (HL)or elastohydrodynamic lubrication (EHL). HL exists when there iscomplete separation of the mating gear teeth during operation achievedby a continuous lubricant film. EHL exists in highly loaded mated gearteeth under operation when the separating fluid film formation isinfluenced by elastic deformation of the contacting surfaces. Hence,with HL or EHL during their high speed and high load operation,superfinished auto racing transmissions experience almost nometal-to-metal contact of the mating teeth.

In sharp contrast to auto racing transmissions, the input planetarystage gears used in the wind turbine power generating industry operateunder significantly different conditions. In wind turbine applications,the gears experience very high, varying loads at low pitch linevelocities such that boundary lubrication rather than hydrodynamic (HL)or elastohydrodynamic lubrication (EHL) is predicted. Boundarylubrication exists when the mating gear teeth during operation arewetted with fluid but the lubricant film thickness is less than thecombined mating surface roughness. Thus, the lubricant film can bepenetrated by peak asperities, and metal-to-metal contact generatesmetal debris from the gear teeth. Traditionally manufactured ground windturbine gear teeth (see “Standard for Design and Specification of GearBoxes for Wind Turbines,” ANSI/AGMA/AWEA 6006-A03) after the run-inprocess described below, are hoped to achieve a surface finish ofR_(a)=0.5-0.7 micron. However, those practiced in the art recognize thata traditionally manufactured hollow wheel will have a much highersurface finish. It is recommended by the AGMA standard that this gear'sfinish not exceed R_(a)>1.6 micron. Finishes of 0.5-0.7 micron areconsidered sufficient to avoid most metal to metal flank contact. It wasalso believed that this surface condition would result in significantlubricant retention needed with the slow moving gear teeth and thus thebest possible lubrication condition would be achieved. However, a majorsource of wind turbine gear box failure is failure of the bearings. Evenwith run-in achieving the above finishes, metal to metal teeth contactcontinues on the planetary gear stage teeth and produces lubricantdebris, which in turn contributes to the rapid bearing failures.

In contrast, chemically accelerated vibratory superfinishing to acondition of Ra<0.3 micron was thought to be too smooth for large windturbine generators in that the teeth flanks would have insufficientlubricant retention for operation and tooth failure was predicted. Thus,it was questionable whether or not superfinishing using chemicallyaccelerated vibratory finishing of the input planetary stage would addany performance value to the gear box. Only lengthy and costly fieldtesting could provide the answer.

In addition, it was thought by those skilled-in-the-art that the large,heavy gears that make up an input planetary stage of a large windturbine generator could not be processed in vibratory finishingequipment used in the chemically accelerated vibratory finishingprocess. This vibratory finishing equipment is either in a bowl or tubform. The input planetary stage gears are typically 200 kg or more forgenerators of an output capacity of 500 kW and larger. This gear weightwas thought to be beyond the normal range of operation for vibratoryfinishing equipment.

In particular, it was thought that a large hollow wheel gear (annulusgear) weighing from 400 kg to greater than 5000 kg could not besuperfinished in a large vibratory bowl. A person skilled-in-the-artwould have predicted that such a massive gear with its relatively smallcross sectional area would have immediately sunk to the bottom of thebowl damaging the lining, the gear or both. In addition, the heavy gearwould have been expected to fracture significant quantities of theceramic media used in the vibratory finishing equipment because of thehigh pressure exerted upon the media. The shards produced by thecrushing of the ceramic media would have sharp points and edges. Insteadof smoothing the critical contact surfaces of the gear teeth to asuperfinished condition, these media fragments would have been predictedto damage these surfaces resulting in roughened, gouged and even dentingthe surfaces, especially nearer the bottom of the bowl where thepressure is greatest. The damage would have been significantly augmentedfor softer through-hardened (32-40 HRC) hollow wheel gears. Theanticipated high rate of media attrition from fracturing would also addan unacceptable processing cost as well as causing the problem ofclogging and blocking the drains of the processing machine.

Additionally, in processing the hollow wheel, it would have beenexpected that there would have been a variance in the intensity of mediapressure across the lead of the gear teeth. The pressure of the media onthe gear teeth nearer the bottom of the bowl is greater than thepressure of the media near the top. As a result, more stock would beexpected to be removed from the gear teeth nearer the bottom than nearerthe top. Therefore, the vibratory processed hollow wheel gear could endup being out of dimensional tolerance. This could be partially mitigatedby removing the gear half way through the process, turning it over,returning it to the bowl, and continuing the process. It should bementioned though that turning such a large gear is time consuming andpotentially dangerous. Also, part of the center width of the gear teethwould be processed for twice the finishing time, possibly causing aresultant change in the tooth geometry. Each of the above predictedshortcomings would have been predicted to make this superfinishingprocess for large hollow wheel gears commercially impractical andunpredictable.

Similar shortcomings would have been expected for the chemicallyaccelerated vibratory finishing of the other gears that make up theinput planetary stage of a wind turbine gear box. These gears, known asplanets and sun pinions, are similarly massive, typically weighing inexcess of 200 kg each. As such, those skilled-in-the-art would havepredicted they could not be processed in vibratory finishing equipment,whether bowls or tubs. Therefore, the wind turbine industry could notrealize the benefits of this superfinishing process for the inputplanetary stage of the gear box.

It should be noted that it is desirous to be able to usethrough-hardened hollow wheel gears instead of gas nitrided or gascarburized hollow wheels in the large input planetary gear stage.Through-hardened hollow wheels are less costly to manufacture.

Gas nitriding is expensive, time consuming, and produces a very hard,brittle “white layer” on the teeth surfaces. Those practiced in the artrecognize this white layer must be removed prior to use of the gear.However, removal of the white layer by grinding is at great expense andrisk to ruining the hollow wheel. Alternative removal of the white layerby chemical dissolution is a very hazardous and environmentallyunfriendly process.

In gas carburizing, due to the significant distortion from the heattreatment process, final grinding of the teeth is required, which isalso an expensive process. Furthermore, after final grinding, the gascarburized hollow wheel requires temper burn inspection, anotherhazardous and environmentally unfriendly process.

Additionally, through-hardened hollow wheels are not just less expensiveto manufacture, they can also be more geometrically accurate whencompared to nitrided or carburized hollow wheels. This is verybeneficial in that the remaining gears of the planetary gear set areroutinely manufactured to high geometrical accuracy. Thus, if a moreaccurate, less expensive through-hardened hollow wheel can be operatedwith high accuracy planet and sun gears, the resulting planetary gearset could be highly efficient and of sufficient durability. If thethrough-hardened hollow wheel could be superfinished using chemicallyaccelerated vibratory finishing, its teeth would be of sufficientsurface capacity and capable of operating in HL or EHL regimes, therebyreducing debris generation. Thus, if superfinished through-hardenedhollow wheels combined with superfinished planets and sun gears canoperate satisfactorily at wind turbine designed loads and speeds, theresult would be a superior input planetary gear stage. Alternatively, ifthe planets and sun pinion gears could be superfinished and mated to anon-superfinished hollow wheel, irrespective of its metallurgical heattreatment, the result would be an improved input planetary gear stagefor a wind turbine generator of output capacity of 500 kW and greater.Therefore, superfinishing some, or preferably all, of the gears in theinput planetary stage will result in a reduction or elimination oflubricant debris generated from the gear teeth, thereby reducing oreliminating a source of damage to the bearings.

In point of fact, gearbox manufacturers for large wind turbine powergenerators had only one viable choice for reducing peak asperities aftergrinding, and that was the run-in process. In the run-in process, thegears are smoothed in the assembled state by operating the gears boxunder various loads and speeds such that the contact area peakasperities shear away or plastic deform themselves. It should bementioned that this was also the most economical route to take as thegearbox has to be tested and certified under load conditions anywayprior to its shipment and installation at its final destination. Therun-in and testing phase can be conducted simultaneously on the sametest rig. The AGMA/AWEA & The Danish Energy Agency, for example, havewritten guidelines for designing wind turbine gearboxes and stress theneed for run-in. The impact of surface finish on gear tooth durabilityis briefly discussed, but the methodology of smoothing the surface isgiven no consideration. This view, that the method of removing the peakasperities is irrelevant, is generally shared by this industry as wellas other gear manufacturers.

An ideal run-in process requires operation of the gearbox at differentloads and speeds to simulate actual field conditions in order to smooththe asperity peaks across the whole load-carrying surface. However,duplicating actual service conditions on a test rig is not onlyvirtually impossible, but is also impractical as well due to equipment,time and cost constraints. During the run-in process, the oil filmthickness is often purposefully reduced to allow more asperity peakcontact thereby resulting in a smoother surface. Once the run-in processis completed, the gearbox run-in lubricant and filtration system shouldbe serviced. Typically the lubricant is drained, the gearbox flushed,and replaced with fresh lubricant, and the filter, which captures metaldebris generated during the run-in process, is cleaned or replaced.Unfortunately, even during run-in, this metal debris can initiateserious damage to the bearings and gear contact surfaces beforecollection in a filter. And, most filters are capable of capturing onlythe largest debris particles and allow passage of the finer particles.These fine particles still are capable of causing surface damage,particularly to the gear box bearings.

Also, no matter how thoroughly or carefully the run-in procedure isconducted, this process leaves microscopic material distress (stressraisers) on the gear teeth contact regions due to the high stressescreated to mechanically shear, fracture or elastically deform the peakasperities. These stress raisers act as initiation sites for futurecontact fatigue failures or micropitting.

Consequently, even after run-in, the input planetary gears oftenexperience micropitting during the early period of field service.Micropitting by itself is another source of metal debris which can causefurther damage to the bearings and gear contact surfaces since the metaldebris is not immediately or completely trapped by the filtrationsystem. It should be stressed that even microscopic metal debrisparticles, which can pass through a 10-micron filter, are still largeenough to initiate damage. Micropitting is acknowledged to be anindicator of possible future gear failure and/or serious wear problems.Whenever severe wear occurs, the gear tooth profile is changed leadingto increased vibration and noise which places an elevated stress on thegear box system.

Additionally, run-in procedures typically only smooth the drive side ofthe hollow wheel and sun gear while leaving the coast sides of thesegear teeth as machined. During adverse operating conditions such asstrong gusts of wind or turbine braking, coast side loading can be highenough to produce asperity contact and contribute to harmful metaldebris. However, chemically accelerated vibratory superfinished gearsare smoothed on both sides of the contact teeth surfaces.

Again, it needs to be emphasized that the industry has failed to giveguidance on the actual optimum surface finish, or on the method ofgenerating such optimum surfaces to improve gearbox durability. Insteadit has relied mainly on run-in procedures to smooth the gear teethcontact areas to what was believed to be a satisfactory condition.

SUMMARY OF THE INVENTION

Disclosed herein is an improved large planetary gear system used on theinput stage of wind turbine power generators. This improved planetarygear system reduces or eliminates lubricant debris traditionallygenerated from the gear teeth, thereby eliminating an initiating sourcefor bearing failure. To achieve these results, some and preferably allof the gear teeth within the planetary gear system are superfinishedusing chemically accelerated vibratory finishing to a surface roughnessof approximately 0.25 micron or less.

In particular, a novel method is disclosed for superfinishing themassive hollow wheel gear, especially a through-hardened hollow wheelgear, placed horizontally in a vibratory bowl.

It will be appreciated that the inventive teachings disclosed herein areuseful to all other applications of large planetary gear sets.Similarly, the teachings of this invention are applicable to some or allthe gears of other large, non-planetary, gear box types where boundarylubrication regimes exist due to traditionally ground teeth finishes.The style of gears within these gear boxes, such as spur, helical, face,bevel and the like, are not important to the inventive concept disclosedherein.

It will be readily apparent to those skilled in this art that variousmodifications and changes of an obvious nature may be made, and all suchmodifications and changes are considered to fall within the scope of theclaims. Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. Examples are superfinishing all thegears, and/or all the bearings within these types of large gear boxes.Hence, the specific embodiments described are not intended to belimiting but merely illustrative of the inventive method.

The unique and significant feature of the process used in the presentinvention is the surface leveling mechanism utilized to achieve thesurface finish. A chemical solution is used in the vibratory bowl or tubin conjunction with ceramic media. When introduced into the machine,this chemical solution reacts with the metal and produces a stable, softconversion coating across the asperities (peaks and valleys) of the gearteeth. The rubbing motion across the flanks of the gear teeth developedby the machine and media effectively rubs the conversion coating off the“peaks” of the surfaces, but leaves the “valleys” untouched. Theconversion coating is continually re-formed and rubbed off during thisstage producing a surface leveling mechanism. This mechanism iscontinued in the vibratory machine until the desired surface finish isattained. At this point, the active chemistry is turned off and istypically rinsed from the machine with a burnish solution, which doesnot react with the basis metal. During this stage, the conversioncoating is rubbed off the gear teeth flanks one final time to producethe finished gears for the input planetary gear stage. And finally,since the process is water based, approximately room temperature andopen atmosphere, there is no chance of tempering of the gear withchemically accelerated vibratory finishing. Thus, temper burn inspectionis not required after superfinishing with the present invention.

Since the asperity peaks are removed prior to installation, nomicro-stresses are introduced such as in the conventional run-insmoothing procedure. In fact, the need for run-in is greatly reduced orcompletely eliminated. Thus the problems of micropitting and fretting ofthe gear surfaces are reduced or eliminated. Also, the gears finishedwith the present invention generate no significant metal debris atstart-up or after being in service for long periods, and thus avoidsmetal debris damaging the bearings. This also allows for longer timebetween lubrication servicing. Since the smoothing of the surfaces alsoreduces friction, the gears do not contribute to the typical break-intemperature spike responsible for a reduced life of the lubricant,bearings and seals. Noise and vibration can also be expected to bereduced for two reasons. Firstly, a reduction of friction will effectreduced vibration and noise. Secondly, a reduction in wear means thatthe transmission error will stay more constant with time, and thereforethe noise also will not increase.

Prior to this invention, attempts to improve the durability of windturbine power generator gearboxes was achieved by surface grinding thegear teeth followed by run-in, whereby the gearbox was operated undervarying loads and speeds. Run-in can remove the peak asperities fromsome of the gear tooth mating surfaces, but also has a number of seriousdeficiencies as discussed above when compared to the present invention.Accordingly, several objects and advantages of the present inventionover the teeth grinding and run-in process applicable to wind turbinepower generators having an individual output power capacity of 500 kWand greater are:

-   -   1. to provide an improved input planetary stage having the        entire teeth flanks superfinished, which reduces or eliminates        damaging metal debris generated by the gears during run-in or        during actual service;    -   2. to provide an improved planetary stage with significantly        reduced or eliminated metal debris normally generated from the        gear teeth, thereby reducing or eliminating an initiating source        for bearing failure;    -   3. to provide a practical and cost-effective method of        superfinishing the large hollow wheel gears, especially high        geometrical accuracy through-hardened hollow wheels, using        chemically accelerated vibratory finishing to a superior surface        having a lower R_(a), an increased R_(mr), and a significant        reduction of stress raisers;    -   4. to provide an improved input planetary stage since now some        and preferably all of the gear flanks of any style gear 200 kg        and larger can be superfinished to an R_(a) of 0.25 micron or        less while maintaining dimensional tolerances;    -   5. to provide an improved input planetary stage with        significantly reduced micropitting and fretting, which can lead        to future macropitting, wear and ultimately to failure of the        teeth and bearings;    -   6. to provide a method which simultaneously superfinishes the        drive and coast sides of all the gears and particularly the        hollow wheel and sun gear teeth again reducing or eliminating        the potential for harmful metal debris;    -   7. to provide an improved input planetary stage with a        significantly reduced temperature spike which can be damaging to        the metallurgy, lubricant and seals during the run in or early        field operation;    -   8. to provide an improved input planetary stage with a        significantly reduced vibration and/or noise caused by friction        and/or tooth profile changes due to wear;    -   9. to provide an improved input planetary stage having gears        with an increased material ratio (R_(mr)) on the contact teeth        surfaces allowing for a greater power density;    -   10. to provide an improved input planetary stage allowing        streamlining or elimination of the run-in process;    -   11. to provide a process that does not require temper burn        inspection after the gears are superfinished;    -   12. to provide a chemically accelerated vibratory finishing        process applicable to all style gears of 200 kg and greater in        all types of gear boxes that operate in a boundary lubrication        regimes such that superfinishing reduces or eliminates lubricant        debris; and    -   13. to provide an improved large gearbox where some, or        preferably, all the gears and/or bearings are superfinished to        reduce or eliminate lubricant debris.

Further objects and advantages will become apparent from a considerationof the ensuing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of the cross section of a planetary gearbox withthree planet gears.

FIG. 2 a is a drawing of a gear tooth illustrating the tooth contactarea.

FIG. 2 b is a drawing of the cross section of two gear teeth.

FIG. 3 is a drawing of the vibratory bowl containing media used tosuperfinish the hollow wheel gear;

FIG. 4 is a drawing of the vibratory bowl illustrating the idealposition for the hollow wheel gear during the superfinishing process.

FIG. 5 is a drawing illustrating the location at which the chemicalsolutions are delivered during the superfinishing process.

FIG. 6 are the surface parameters and profile measured on a typicalmachined/ground flank of a hollow wheel gear tooth.

FIG. 7 are the surface parameters and profile measured on a typicalsuperfinished flank of a hollow wheel gear tooth using the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a drawing of an input planetary stage typically used in windturbine gearboxes. It consists of a hollow wheel gear (1), two or moreplanet gears (2), and a sun gear (3). The teeth (4) of each gear are tobe superfinished. FIG. 2 a is a 3-dimensional view of a single geartooth (4) and FIG. 2 b shows a 2-dimensional cross section of two gearteeth (4). The gear tooth (4) consists of the flank (5), which is theworking or contacting side of the gear tooth, the top land (6), which isthe top surface of the gear tooth, the bottom land (7), which is thesurface at the bottom of the space between adjacent teeth, and the rootfillet (8), which is the rounded portion at the base of the gear toothbetween the tooth flank (5) and the bottom land (7). The most criticalarea is the tooth contact pattern (9), which is the surface area of agear tooth which has been in contact with its mate when the gears are inoperation. In the present invention, one or more of the planetary gearsof a wind turbine gearbox including the sun (3), the planets (2) and thehollow wheel gear (1) are superfinished on their drive and coast sidesusing chemically accelerated finishing in a vibratory bowl or tub to anR_(a) of 0.25 micron or less.

General Description of Superfinishing Process

A general description of the superfinishing process follow with commonlyowned U.S. Pat. Nos. 4,491,500 and 4,818,333, and U.S. patentapplication Ser. Nos. 10/071,533, 09/758,067, and 10/684,073, each ofwhich is incorporated herein by reference. An active chemistry isintroduced into the vibratory finishing apparatus that is capable ofconverting the metal of the gear to a composition of a reduced hardnessfilm that is physically and chemically stable and may or may not bevisually perceptible. This film is known as a conversion coating. Whenthis film is developed on the surface of the gear, the action of themedia elements upon the gear will only remove the film from the asperitypeaks of the gear, leaving the depressed areas of the coating intact. Byconstantly wetting the metal surface with the active chemistry, thestable coating will continuously re-form, covering those areas where thebare underlying metal has been freshly exposed, to provide a new layerof the relatively soft film. If that portion remains higher than theadjacent areas it will continue to be rubbed away until any roughnesshas been virtually eliminated.

The amount of active chemistry solution utilized will be only that whichwill maintain all surfaces of the treated parts in a wetted condition,so as to ensure continuous and virtually instantaneous reformation ofany coating removed through the rubbing action. As will be evident tothose skilled-in-the-art, the amount of any media utilized will dependupon numerous factors, such as the surface character, area, weight andcomposition of the gears being treated, the composition of the solutionutilized for the conversion coating, temperatures of operation, thedegree and rate of refinement to be achieved, etc.

Although the properties exhibited by the conversion coating produced onthe gear are of crucial importance to the successful practice of thepresent process, the formulation of the active chemistry utilized toproduce the coating is not. The composition must be capable of quicklyand effectively producing, under the conditions of operation, relativelysoft reaction products of the basis metal and the coating must besubstantially insoluble in the liquid medium so as to ensure thatremoval occurs primarily by rubbing, rather than by dissolution. Theactive chemistry will generally consist of water and up to about 40weight percent of active ingredients, comprised essentially theconversion chemicals but also optionally and desirably including anoxidizing agent, and in some instances a stabilizer and/or a wettingagent. After the desired amount of refinement is attained, the activechemistry is shut off. Thereafter, a burnishing solution may beintroduced into the vibratory machine. The burnishing solution, which isnon-reactive to the basis metal, serves to remove the conversion coatingfrom the surface to create a specular appearance.

Superfinishing the Planets and Sun Gears

In one embodiment of the present invention, the sun gear and planetgears can be superfinished in a suitably sized vibratory bowl or tubmachines. Multiple gears with suitable mounting can be superfinishedsimultaneously. A device may be used to support the gear(s) or to keepthe gear(s) from contacting the sides of the vibratory bowl or tub whilein operation. The gear(s) are rapidly agitated to produce relativemovement among the gear(s) and the non-abrasive ceramic media. Thesurfaces of the gear(s) and the media are maintained in a wettedcondition with an aqueous solution of FERROMIL® FML-590 at 30 v/v %. Thenon-abrasive solid media elements are of an amount, size and shape suchthat, under the conditions of agitation, produce uniform media rubbingof the gear teeth. The process is continued until the arithmetic averageroughness (R_(a)) value is 0.25 micron or less. The gear(s) are thenburnished to remove the conversion coating using an aqueous solution of1.5 v/v % of FERROMIL® FBC-295 to a specular appearance.

While the preferred embodiment contemplates the use of non-abrasiveceramic media, other ceramic media, plastic media, steel media,stainless steel media and combinations of different types of media, canalso be used, depending upon the physical circumstances surrounding thefinishing of the gear. It is within the skill of one in the art todetermine which media, or combination of media, to use in each instance.

Superfinishing the Hollow Wheel Gear

This example teaches one embodiment for superfinishing a large hollowwheel gear (1) suitable for commercial wind turbine gearboxes of outputpower capacity of 500 kW and greater. The hollow wheel gear (1) has thefollowing approximate weight and dimensions. It weighs 1,620 kg, has anouter diameter of 171 cm, an inner diameter of 146 cm, and a face widthof 38.5 cm. The hollow wheel can be heat treated via gas carburization,gas nitriding, or it can be through hardening. In FIG. 3, a vibratorybowl (10) is filled to approximately two-thirds of its volume with amixture of abrasive and non-abrasive ceramic media (11). The media sizeand shapes are selected such that a homogenous mixture of media hasuniform contact across the gear tooth flank. The amount of media is alsochosen to give the preferred amount of lifting action during processingsuch that the gear does not contact the bottom or sides of the vibratorybowl channel, or such that the top of the gear does not climb above theworking media level. The motor weights are set to a lead angle ofapproximately 85 degrees.

The hollow wheel gear (1) is laid horizontally over the center hub (12)of the vibratory bowl (10) onto the stationary media mass (11) takingreasonable care to center the hollow wheel gear relative to the centerof the bowl. As illustrated in FIG. 5 an aqueous solution of FERROMIL®FBC-295 at 1.5 v/v % with a flow rate of 20 L/hr is delivered into theregion between the outside wall of the bowl and the outer surface of thegear (13) to reduce the effects of frictional heat generation. Anaqueous solution of active chemistry consisting of FERROMIL® FML-590 at30 v/v % is delivered at 18 L/hr into the region between the center hub(12) and the internal gear teeth (14).

The vibratory bowl (10) is started at a low frequency and is graduallyincreased to approximately 46 to 48 hertz whereby the hollow wheel gearsettles into the media (11). The ideal position is shown in FIG. 4 wherethe uppermost part of the gear (1) is at or just below the media (11)and air interface. If the vibratory bowl amplitude is not between 1.5 to2.0 mm, adjustments should be made to the lower weight to attain thisamplitude. This measurement is read from an amplitude gauge mounted onthe outside of the bowl (10). The hollow wheel gear (1) will remaincentered during the remainder of the processing and will slowly rotatearound the center hub of the vibratory bowl. (12).

The following parameters may be adjusted as needed in order to keep thegear (1) at or just below the upper surface of the media (11) so that itrotates uniformly around the center hub (12) of the vibratory bowl (10):

-   -   Media size, shape, composition and percentage of each.    -   Media level.    -   Frequency of the motor.    -   Amplitude and lead angle generated by the adjustable weight        system.    -   Concentrations and flow rates of active chemistry and burnish        solutions.        The adjustment of these parameters is within the knowledge of        one of ordinary skill of the art.

The process is continued until the arithmetic average roughness (R_(a))value of 0.25 micron or less. The flow of active chemistry is shut off,and a burnishing compound consisting of an aqueous solution of FERROMIL®FBC-295 at 1.5 v/v % is delivered at 150 L/hr into the region betweenthe center column of the bowl and the teeth of the gear (15). Theprocess is continued until the conversion coating is removed producing aclean and bright appearance.

The unanticipated results that were obtained were:

-   -   1. The gear remains centered in the bowl, and is suspended off        the bottom of the bowl by the motion of the media, and the        uppermost part of the gear remains at or just below the        media/air interface.    -   2. The gear is superfinished with no damage from the media or        media shards.    -   3. An R_(a) 0.25 micron or less is achieved, and the material        ratio is significantly increased.        -   a. FIG. 6 shows a typical surface roughness profile of the            gear teeth contact area prior to superfinishing. The R_(a)            is 0.78 micron, and the R_(mr) is 49.4%.        -   b. FIG. 7 shows the surface roughness profile of the gear            teeth contact area after superfinishing. The R_(a) is 0.16            micron, and the R_(mr) is 73.2%    -   4. The surface finish is uniform, within tolerances, across the        lead and profile.    -   5. Only an insignificant amount, if any, of media is fractured        by the process (i.e., the media attrition was extremely low).

Planetary Testing in the Field

Two wind turbine generator gearboxes having an output power capacity ofgreater than 500 kW had all of the gears from the input planetary stagesuperfinished to a surface roughness of 0.25 micron or less using theprocess described in the present invention. Prior to superfinishing, thehollow wheels were through-hardened, and the planets and sun gears weregas carburized. After being placed in service, the gearboxes wereinspected after approximately 6 months and after approximately one yearof operation. No micropitting or fretting was observed on the gear teethsurfaces. Similarly, no bearing damage was found. In comparison, groundgears smoothed only by the run-in technique commonly can start to showsigns of micropitting or fretting after only 6 months of operation, andbearings begin to show damage via direct inspection or bynoise/vibration monitoring. Further anticipated advantages of thepresent invention include reduced metal debris, improved bearing life,reduced wear, reduced vibration and noise, improved contact fatigueresistance, improved lubrication, increased time between lubricantservicing, a simplified or eliminated run-in process, and enhanceddurability, efficiency, and reduced manufacturing and operating cost ofthe planetary gearbox.

While the apparatuses and methods of the present invention have beendescribed in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to what hasbeen described herein without departing from the concept and scope ofthe invention. All such similar substitutes and modifications apparentto those skilled in the art are deemed to be within the scope andconcept of the invention, in particular the applicability of thisprocess to finishing any type gear, any type of large planetary gearsystem, not just those involved in the wind turbine industry and anytype of large gear box having individual gears greater than 200 kg thatoperate in boundary lubrication regimes. Such examples of otherindustries in which this technology will be useful is in the marinepropulsion and earth moving industries, the mining industries, as wellas any other industry employing the use of large gear systems.

1. A planetary gearbox comprising a hollow wheel gear, two or moreplanet gears, and a sun gear, wherein one or more of the hollow wheelgear, two or more planet gears, and sun gear comprise a plurality ofteeth that have been superfinished to a surface roughness sufficient toreduce lubricant debris buildup in the planetary gearbox.
 2. Theplanetary gearbox of claim 1, wherein one or more of the hollow wheelgear, two or more planet gears, and sun gear are superfinished usingchemically accelerated finishing.
 3. The planetary gearbox of claim 1,wherein the teeth of one or more of the hollow wheel gear, two or moreplanet gears, and a sun gear are superfinished to a final surfaceroughness of 0.25 μm or less.
 4. The planetary gearbox of claim 1,wherein the hollow wheel gear is superfinished by agitating the hollowwheel gear in the presence of a plurality of finishing media and achemical solution that is capable of reacting with the metal of thehollow wheel gear to form a film having a reduced hardness film on thesurface of the hollow wheel gear, so that the plurality of finishingmedia can remove the reduced hardness film from the surface of thehollow wheel gear, thereby refining the surface of the hollow wheelgear, after which the composition having a reduced hardness film isimmediately re-formed by the reaction between the hollow wheel gear andthe chemical solution for further refining by the plurality of finishingmedia.
 5. The planetary gearbox of claim 4, wherein the plurality offinishing media is selected from the group consisting of abrasive media,non-abrasive media, and mixtures thereof.
 6. The planetary gearbox ofclaim 5, wherein the abrasive media is selected from the groupconsisting of quartz, granite, natural and synthetic aluminim oxides,silicon carbide, iron oxides, and mixtures thereof.
 7. The planetarygearbox of claim 6, wherein the abrasive media is held within a matrixof porcelain, plastic, or mixtures thereof.
 8. The planetary gearbox ofclaim 5, wherein the non-abrasive media is selected from the groupconsisting of ceramic media, plastic media, steel media, stainless steelmedia, and mixtures thereof.
 9. The planetary gearbox of claim 4,wherein the mixture occurs within a vibratory finishing apparatus. 10.The planetary gearbox of claim 4, wherein the amount of chemicalsolution is sufficient to maintain the surfaces of the hollow wheel gearand plurality of finishing media in a wetted condition.
 11. Theplanetary gearbox of claim 4, wherein the chemical solution comprises achemical selected from the group consisting of phosphate salt, phosphateacid, oxalic acid, sodium oxalate, sulfate, sulfuric acid, sodiumbicarbonate, chromate, chromic acid, sodium chromate, or mixturesthereof.
 12. The planetary gearbox of claim 4, wherein the chemicalsolution comprises an accelerator selected from the group consisting ofzinc, magnesium phosphates, iron phosphates, organic oxidizers,inorganic oxidizers, peroxides, meta-nitrobenzene, chlorates, chlorites,nitrates, nitrites, and mixtures thereof.
 13. The planetary gearbox ofclaim 1, wherein the planet gears are superfinished by agitating theplanet gears in the presence of a plurality of finishing media and achemical solution that is capable of reacting with the metal of theplanet gears to form a film having a reduced hardness film on thesurface of the planet gears, so that the plurality of finishing mediacan remove the reduced hardness film from the surface of the planetgears, thereby refining the surface of the planet gears, after which thecomposition having a reduced hardness film is immediately re-formed bythe reaction between the planet gears and the chemical solution forfurther refining by the plurality of finishing media.
 14. The planetarygearbox of claim 13, wherein the plurality of finishing media isselected from the group consisting of abrasive media, non-abrasivemedia, and mixtures thereof.
 15. The planetary gearbox of claim 14,wherein the non-abrasive media comprises a mixture of oxide grains fusedto a coherent mass and substantially free of discrete abrasiveparticles, the coherent mass containing, on an oxygen-free basis, about60 to 80 weight percent aluminum and about 5 to 30 weight silicon. 16.The planetary gearbox of claim 15 wherein the coherent mass comprisesabout 76 to 78 weight percent aluminum, about 10 to 12 weight percentsilicon, about 5 to 9 weight percent iron and about 4 to 6 weightpercent titanium, on an oxygen-free basis.
 17. The planetary gearbox ofclaim 15 wherein the coherent mass comprises about 63 to 67 weightpercent aluminum, about 26 to 36 weight percent silicon, about 2 to 4weight percent sodium, about 1 to 2 weight percent potassium, and about0.5 to 0.8 weight percent phosphorous, on an oxygen-free basis.
 18. Theplanetary gearbox of claim 15 wherein the coherent mass comprises about62 to 73 weight percent aluminum, about 7 to 14 weight percent silicon,about 10 to 25 weight percent manganese, and about 1 to 4 weight percentsodium.
 19. The planetary gearbox of claim 15 wherein the oxide grainshave diameters between 1 micron and 25 microns.
 20. The planetarygearbox of claim 14, wherein the non-abrasive media has a density of atleast about 2.75 grams per cubic centimeter.
 21. The planetary gearboxof claim 14, wherein the non-abrasive media has an average diamondpyramid hardness value of at least about
 845. 22. The planetary gearboxof claim 14, wherein the average weight reduction of the plurality offinishing media caused by their agitation in the process will not exceedabout 0.1 percent per hour.
 23. The planetary gearbox of claim 13,wherein the amount of chemical solution is sufficient to maintain thesurfaces of the hollow wheel gear and plurality of finishing media in awetted condition.
 24. The planetary gearbox of claim 13, wherein thechemical solution comprises a chemical selected from the groupconsisting of phosphate salt, phosphate acid, oxalic acid, sodiumoxalate, sulfate, sulfuric acid, sodium bicarbonate, chromate, chromicacid, sodium chromate, or mixtures thereof.
 25. The planetary gearbox ofclaim 13, wherein the chemical solution comprises an acceleratorselected from the group consisting of zinc, magnesium phosphates, ironphosphates, organic oxidizers, inorganic oxidizers, peroxides,meta-nitrobenzene, chlorates, chlorites, nitrates, nitrites, andmixtures thereof.
 26. The planetary gearbox of claim 1, wherein the sungear is superfinished by agitating the sun gear in the presence of aplurality of finishing media and a chemical solution that is capable ofreacting with the metal of the sun gear to form a film having a reducedhardness film on the surface of the sun gear, so that the plurality offinishing media can remove the reduced hardness film from the surface ofthe sun gear, thereby refining the surface of the sun gear, after whichthe composition having a reduced hardness film is immediately re-formedby the reaction between the sun gear and the chemical solution forfurther refining by the plurality of finishing media.
 27. The planetarygearbox of claim 26, wherein the plurality of finishing media isselected from the group consisting of abrasive media, non-abrasivemedia, and mixtures thereof.
 28. The planetary gearbox of claim 27,wherein the non-abrasive media comprises a mixture of oxide grains fusedto a coherent mass and substantially free of discrete abrasiveparticles, the coherent mass containing, on an oxygen-free basis, about60 to 80 weight percent aluminum and about 5 to 30 weight silicon. 29.The planetary gearbox of claim 28 wherein the coherent mass comprisesabout 76 to 78 weight percent aluminum, about 10 to 12 weight percentsilicon, about 5 to 9 weight percent iron and about 4 to 6 weightpercent titanium, on an oxygen-free basis.
 30. The planetary gearbox ofclaim 28 wherein the coherent mass comprises about 63 to 67 weightpercent aluminum, about 26 to 36 weight percent silicon, about 2 to 4weight percent sodium, about 1 to 2 weight percent potassium, and about0.5 to 0.8 weight percent phosphorous, on an oxygen-free basis.
 31. Theplanetary gearbox of claim 28 wherein the coherent mass comprises about62 to 73 weight percent aluminum, about 7 to 14 weight percent silicon,about 10 to 25 weight percent manganese, and about 1 to 4 weight percentsodium.
 32. The planetary gearbox of claim 28 wherein the oxide grainshave diameters between 1 micron and 25 microns.
 33. The planetarygearbox of claim 27, wherein the non-abrasive media has a density of atleast about 2.75 grams per cubic centimeter.
 34. The planetary gearboxof claim 27, wherein the non-abrasive media has an average diamondpyramid hardness value of at least about
 845. 35. The planetary gearboxof claim 26, wherein the average weight reduction of the plurality offinishing media caused by their agitation in the process will not exceedabout 0.1 percent per hour.
 36. The planetary gearbox of claim 26,wherein the amount of chemical solution is sufficient to maintain thesurfaces of the hollow wheel gear and plurality of finishing media in awetted condition.
 37. The planetary gearbox of claim 26, wherein thechemical solution comprises a chemical selected from the groupconsisting of phosphate salt, phosphate acid, oxalic acid, sodiumoxalate, sulfate, sulfuric acid, sodium bicarbonate, chromate, chromicacid, sodium chromate, or mixtures thereof.
 38. The planetary gearbox ofclaim 26, wherein the chemical solution comprises an acceleratorselected from the group consisting of zinc, magnesium phosphates, ironphosphates, organic oxidizers, inorganic oxidizers, peroxides,meta-nitrobenzene, chlorates, chlorites, nitrates, nitrites, andmixtures thereof.
 39. The planetary gearbox of claim 1, wherein theplanetary gearbox a planetary gearbox taken from a wind turbine unit, amarine propulsion unit, or an earth moving unit.
 40. The planetarygearbox of claim 1, wherein prior to being superfinished to a surfaceroughness of 0.25 μm or less, the one or more of the hollow wheel gear,two or more planet gears, and sun gear are heat treated by a methodselected from the group consisting of gas carburization, gas nitriding,and through hardening.
 41. The planetary gearbox of claim 1, whereineach of the hollow wheel gear, two or more planet gears, and sun gearcomprise a plurality of teeth that have been superfinished to a surfaceroughness of 0.25 μm or less.
 42. The planetary gearbox of claim 1,wherein each of the two or more planet gears and sun gear comprise aplurality of teeth that have been superfinished to a surface roughnessof 0.25 μm or less.
 43. The planetary gearbox of claim 1, wherein eachof the two or more planet gears comprise a plurality of teeth that havebeen superfinished to a surface roughness of 0.25 μm or less.
 44. Theplanetary gearbox of claim 1, wherein the drive side of the plurality ofteeth have been superfinished to a surface roughness of 0.25 μm or less.45. The planetary gearbox of claim 1, wherein the coast side of theplurality of teeth have been superfinished to a surface roughness of0.25 μm or less.
 46. A gear having a mass of greater than 200 kg thatcomprises a plurality of teeth that have been superfinished to a surfaceroughness of 0.25 μm or less.
 47. The gear of claim 46, wherein the gearhaving a mass of greater than 200 kg is superfinished using chemicallyaccelerated finishing.
 48. The gear of claim 46, wherein the gear havinga mass of greater than 200 kg is superfinished by agitating the gearhaving a mass of greater than 200 kg in the presence of a plurality offinishing media and a chemical solution that is capable of reacting withthe metal of the gear having a mass of greater than 200 kg to form afilm having a reduced hardness film on the surface of the gear having amass of greater than 200 kg, so that the plurality of finishing mediacan remove the reduced hardness film from the surface of the gear havinga mass of greater than 200 kg, thereby refining the surface of the gearhaving a mass of greater than 200 kg, after which the composition havinga reduced hardness film is immediately re-formed by the reaction betweenthe gear having a mass of greater than 200 kg and the chemical solutionfor further refining by the plurality of finishing media.
 49. The gearof claim 48, wherein the plurality of finishing media is selected fromthe group consisting of abrasive media, non-abrasive media, and mixturesthereof.
 50. The gear of claim 49, wherein the abrasive media isselected from the group consisting of quartz, granite, natural andsynthetic aluminum oxides, silicon carbide, iron oxides, and mixturesthereof.
 51. The gear of claim 50, wherein the abrasive media is heldwithin a matrix of porcelain, plastic, or mixtures thereof.
 52. The gearof claim 49, wherein the non-abrasive media is selected from the groupconsisting of ceramic media, plastic media, steel media, stainless steelmedia, and mixtures thereof.
 53. The gear of claim 48, wherein themixture occurs within a vibratory finishing apparatus.
 54. The gear ofclaim 48, wherein the amount of chemical solution is sufficient tomaintain the surfaces of the hollow wheel gear and plurality offinishing media in a wetted condition.
 55. The gear of claim 48, whereinthe chemical solution comprises a chemical selected from the groupconsisting of phosphate salt, phosphate acid, oxalic acid, sodiumoxalate, sulfate, sulfuric acid, sodium bicarbonate, chromate, chromicacid, sodium chromate, or mixtures thereof.
 56. The gear of claim 48,wherein the chemical solution comprises an accelerator selected from thegroup consisting of zinc, magnesium phosphates, iron phosphates, organicoxidizers, inorganic oxidizers, peroxides, meta-nitrobenzene; chlorates,chlorites, nitrates, nitrites, and mixtures thereof.
 57. The gear ofclaim 46, wherein the drive side of the plurality of teeth have beensuperfinished to a surface roughness of 0.25 μm or less.
 58. The gear ofclaim 46, wherein the coast side of the plurality of teeth have beensuperfinished to a surface roughness of 0.25 μm or less.
 59. A methodfor reducing lubricant debris in a planetary gearbox, wherein theplanetary gearbox comprises a hollow wheel gear, two or more planetgears, and a sun gear, the method comprising superfinishing the teeth ofone or more of the hollow wheel gear, two or more planet gears, and sungear to a surface roughness of 0.25 μm or less before operating theplanetary gearbox.
 60. The method of claim 59, wherein one or more ofthe hollow wheel gear, two or more planet gears, and sun gear aresuperfinished using chemically accelerated finishing.
 61. The method ofclaim 59, wherein prior to being superfinished to a surface roughness of0.25 μm or less, the one or more of the hollow wheel gear, two or moreplanet gears, and sun gear are heat treated by a method selected fromthe group consisting of gas carburization, gas nitriding, and throughhardening.
 62. The method of claim 59, wherein each of the hollow wheelgear, two or more planet gears, and sun gear comprise a plurality ofteeth that have been superfinished to a surface roughness of 0.25 μm orless.
 63. The method of claim 59, wherein each of the two or more planetgears and sun gear comprise a plurality of teeth that have beensuperfinished to a surface roughness of 0.25 μm or less.
 64. The methodof claim 59, wherein each of the two or more planet gears comprise aplurality of teeth that have been superfinished to a surface roughnessof 0.25 μm or less.
 65. The method of claim 59, wherein the drive sideof the plurality of teeth have been superfinished to a surface roughnessof 0.25 μm or less.
 66. The method of claim 59, wherein the coast sideof the plurality of teeth have been superfinished to a surface roughnessof 0.25 μm or less.